diff --git a/hudi-client/hudi-client-common/src/main/java/org/apache/hudi/index/bucket/BucketIdentifier.java b/hudi-client/hudi-client-common/src/main/java/org/apache/hudi/index/bucket/BucketIdentifier.java
index 4ab36d6a5..ddd95721a 100644
--- a/hudi-client/hudi-client-common/src/main/java/org/apache/hudi/index/bucket/BucketIdentifier.java
+++ b/hudi-client/hudi-client-common/src/main/java/org/apache/hudi/index/bucket/BucketIdentifier.java
@@ -23,6 +23,7 @@ import org.apache.hudi.common.model.HoodieKey;
 import org.apache.hudi.common.model.HoodieRecord;
 
 import java.util.Arrays;
+import java.util.Collections;
 import java.util.List;
 import java.util.Map;
 import java.util.regex.Pattern;
@@ -39,7 +40,7 @@ public class BucketIdentifier {
   public static int getBucketId(HoodieKey hoodieKey, String indexKeyFields, int numBuckets) {
     List<String> hashKeyFields;
     if (!hoodieKey.getRecordKey().contains(":")) {
-      hashKeyFields = Arrays.asList(hoodieKey.getRecordKey());
+      hashKeyFields = Collections.singletonList(hoodieKey.getRecordKey());
     } else {
       Map<String, String> recordKeyPairs = Arrays.stream(hoodieKey.getRecordKey().split(","))
           .map(p -> p.split(":"))
diff --git a/hudi-flink/src/main/java/org/apache/hudi/sink/BucketStreamWriteFunction.java b/hudi-flink/src/main/java/org/apache/hudi/sink/BucketStreamWriteFunction.java
index ac5dd630b..128358096 100644
--- a/hudi-flink/src/main/java/org/apache/hudi/sink/BucketStreamWriteFunction.java
+++ b/hudi-flink/src/main/java/org/apache/hudi/sink/BucketStreamWriteFunction.java
@@ -41,6 +41,15 @@ import java.util.List;
 
 import static java.util.stream.Collectors.toList;
 
+/**
+ * A stream write function with bucket hash index.
+ *
+ * <p>The task holds a fresh new local index: {(partition + bucket number) &rarr fileId} mapping, this index
+ * is used for deciding whether the incoming records in an UPDATE or INSERT.
+ * The index is local because different partition paths have separate items in the index.
+ *
+ * @param <I> the input type
+ */
 public class BucketStreamWriteFunction<I> extends StreamWriteFunction<I> {
 
   private static final Logger LOG = LoggerFactory.getLogger(BucketStreamWriteFunction.class);
@@ -51,11 +60,11 @@ public class BucketStreamWriteFunction<I> extends StreamWriteFunction<I> {
 
   private int bucketNum;
 
-  protected transient HoodieFlinkTable table;
+  private transient HoodieFlinkTable<?> table;
 
   private String indexKeyFields;
 
-  private HashMap<String, String> bucketToFileIDMap;
+  private final HashMap<String, String> bucketToFileIDMap;
 
   /**
    * Constructs a BucketStreamWriteFunction.
@@ -64,7 +73,7 @@ public class BucketStreamWriteFunction<I> extends StreamWriteFunction<I> {
    */
   public BucketStreamWriteFunction(Configuration config) {
     super(config);
-    this.bucketToFileIDMap = new HashMap<String, String>();
+    this.bucketToFileIDMap = new HashMap<>();
   }
 
   @Override
@@ -127,7 +136,7 @@ public class BucketStreamWriteFunction<I> extends StreamWriteFunction<I> {
         bucketToLoad.add(i);
       }
     }
-    bucketToLoad.stream().forEach(bucket -> LOG.info(String.format("bucketToLoad contains %s", bucket)));
+    bucketToLoad.forEach(bucket -> LOG.info(String.format("bucketToLoad contains %s", bucket)));
 
     LOG.info(String.format("Loading Hoodie Table %s, with path %s", table.getMetaClient().getTableConfig().getTableName(),
         table.getMetaClient().getBasePath()));
@@ -146,7 +155,7 @@ public class BucketStreamWriteFunction<I> extends StreamWriteFunction<I> {
           LOG.info(String.format("Should load this partition bucket %s with fileID %s", partitionBucketId, fileID));
           if (bucketToFileIDMap.containsKey(partitionBucketId)) {
             throw new RuntimeException(String.format("Duplicate fileID %s from partitionBucket %s found "
-              + "during the fileGroupPerPartitionedBucketState initialization.", fileID, partitionBucketId));
+              + "during the BucketStreamWriteFunction index bootstrap.", fileID, partitionBucketId));
           } else {
             LOG.info(String.format("Adding fileID %s to the partition bucket %s.", fileID, partitionBucketId));
             bucketToFileIDMap.put(partitionBucketId, fileID);
diff --git a/hudi-flink/src/main/java/org/apache/hudi/sink/BucketStreamWriteOperator.java b/hudi-flink/src/main/java/org/apache/hudi/sink/BucketStreamWriteOperator.java
index cf4ef3f78..209fe59e4 100644
--- a/hudi-flink/src/main/java/org/apache/hudi/sink/BucketStreamWriteOperator.java
+++ b/hudi-flink/src/main/java/org/apache/hudi/sink/BucketStreamWriteOperator.java
@@ -22,6 +22,11 @@ import org.apache.flink.configuration.Configuration;
 import org.apache.hudi.sink.common.AbstractWriteOperator;
 import org.apache.hudi.sink.common.WriteOperatorFactory;
 
+/**
+ * Operator for {@link BucketStreamWriteFunction}.
+ *
+ * @param <I> The input type
+ */
 public class BucketStreamWriteOperator<I> extends AbstractWriteOperator<I> {
 
   public BucketStreamWriteOperator(Configuration conf) {
diff --git a/hudi-flink/src/main/java/org/apache/hudi/sink/partitioner/BucketIndexPartitioner.java b/hudi-flink/src/main/java/org/apache/hudi/sink/partitioner/BucketIndexPartitioner.java
index aab527185..ab46b0317 100644
--- a/hudi-flink/src/main/java/org/apache/hudi/sink/partitioner/BucketIndexPartitioner.java
+++ b/hudi-flink/src/main/java/org/apache/hudi/sink/partitioner/BucketIndexPartitioner.java
@@ -22,7 +22,13 @@ import org.apache.flink.api.common.functions.Partitioner;
 import org.apache.hudi.common.model.HoodieKey;
 import org.apache.hudi.index.bucket.BucketIdentifier;
 
-public class BucketIndexPartitioner implements Partitioner {
+/**
+ * Bucket index input partitioner.
+ * The fields to hash can be a subset of the primary key fields.
+ *
+ * @param <T> The type of obj to hash
+ */
+public class BucketIndexPartitioner<T extends HoodieKey> implements Partitioner<T> {
 
   private final int bucketNum;
   private final String indexKeyFields;
@@ -33,8 +39,8 @@ public class BucketIndexPartitioner implements Partitioner {
   }
 
   @Override
-  public int partition(Object key, int numPartitions) {
-    int curBucket = BucketIdentifier.getBucketId((HoodieKey) key, indexKeyFields, bucketNum);
+  public int partition(HoodieKey key, int numPartitions) {
+    int curBucket = BucketIdentifier.getBucketId(key, indexKeyFields, bucketNum);
     return BucketIdentifier.mod(curBucket, numPartitions);
   }
 }
diff --git a/hudi-flink/src/main/java/org/apache/hudi/sink/utils/Pipelines.java b/hudi-flink/src/main/java/org/apache/hudi/sink/utils/Pipelines.java
index 09b7369ed..ae8b4f213 100644
--- a/hudi-flink/src/main/java/org/apache/hudi/sink/utils/Pipelines.java
+++ b/hudi-flink/src/main/java/org/apache/hudi/sink/utils/Pipelines.java
@@ -18,6 +18,7 @@
 
 package org.apache.hudi.sink.utils;
 
+import org.apache.hudi.common.model.HoodieKey;
 import org.apache.hudi.common.model.HoodieRecord;
 import org.apache.hudi.configuration.FlinkOptions;
 import org.apache.hudi.configuration.OptionsResolver;
@@ -276,10 +277,10 @@ public class Pipelines {
       WriteOperatorFactory<HoodieRecord> operatorFactory = BucketStreamWriteOperator.getFactory(conf);
       int bucketNum = conf.getInteger(FlinkOptions.BUCKET_INDEX_NUM_BUCKETS);
       String indexKeyFields = conf.getString(FlinkOptions.INDEX_KEY_FIELD);
-      BucketIndexPartitioner partitioner = new BucketIndexPartitioner(bucketNum, indexKeyFields);
+      BucketIndexPartitioner<HoodieKey> partitioner = new BucketIndexPartitioner<>(bucketNum, indexKeyFields);
       return dataStream.partitionCustom(partitioner, HoodieRecord::getKey)
-        .transform("hoodie_bucket_stream_write", TypeInformation.of(Object.class), operatorFactory)
-        .uid("uid_hoodie_bucket_stream_write" + conf.getString(FlinkOptions.TABLE_NAME))
+        .transform("bucket_write", TypeInformation.of(Object.class), operatorFactory)
+        .uid("uid_bucket_write" + conf.getString(FlinkOptions.TABLE_NAME))
         .setParallelism(conf.getInteger(FlinkOptions.WRITE_TASKS));
     } else {
       WriteOperatorFactory<HoodieRecord> operatorFactory = StreamWriteOperator.getFactory(conf);
@@ -294,8 +295,8 @@ public class Pipelines {
         .setParallelism(conf.getOptional(FlinkOptions.BUCKET_ASSIGN_TASKS).orElse(defaultParallelism))
         // shuffle by fileId(bucket id)
         .keyBy(record -> record.getCurrentLocation().getFileId())
-        .transform("hoodie_stream_write", TypeInformation.of(Object.class), operatorFactory)
-        .uid("uid_hoodie_stream_write" + conf.getString(FlinkOptions.TABLE_NAME))
+        .transform("stream_write", TypeInformation.of(Object.class), operatorFactory)
+        .uid("uid_stream_write" + conf.getString(FlinkOptions.TABLE_NAME))
         .setParallelism(conf.getInteger(FlinkOptions.WRITE_TASKS));
     }
   }